1. Field of the Invention
The present invention relates to an exhaust emission purifying apparatus for an engine, and in particular, to a technology for purifying nitrogen oxides emitted from the engine by using ammonia as a reducing agent.
2. Description of the Related Art
There has been known the following SCR (Selective Catalytic Reduction) apparatus, as an apparatus for purifying, by the post-treatment, the air pollution offender emitted from an engine, in particular, nitrogen oxides (to be referred to as “NOR”) in the exhaust gas. In this apparatus, an injection device for an aqueous solution of ammonia or a precursor thereof is disposed in an exhaust passage of the engine, and injected ammonia is used as a reducing agent to be reacted with NOx on a catalyst, to thereby reduce and purify NOR. Further, there has also been known a SCR apparatus in which, taking the ammonia storability on a vehicle into consideration, urea being the ammonia precursor is stored in a tank in an aqueous solution state, and in actual operations, the urea water supplied from the tank is injected into an exhaust passage, to thereby generate ammonia by the urea hydrolysis utilizing the exhaust heat (refer to Japanese Unexamined Patent Publication No. 2000-027627, paragraph number 0013).
The applicant of this invention has reviewed the application of the SCR apparatus to the exhaust gas purification in an on-vehicle engine. In order to inject the urea water of amount precise for a NO emission amount from the engine and to satisfactorily perform the NO reduction-reaction, it is important from a practical view point that a urea sensor is disposed in a urea water tank, and the actual urea concentration (hereunder, the simply called “concentration” indicates the urea concentration) is reflected onto controlling of the engine and the SCR apparatus. At the present day, there has been developed a urea sensor in which a heater and a resistance temperature sensor are disposed in an insulation state, and focusing attention on a heat transfer characteristic of the urea water according to the urea concentration, the actual urea concentration is detected based on an electric resistance value of the resistance temperature sensor (refer to Japanese Unexamined Patent Publication No. 2001-228004).
The applicant of this invention has already disclosed an exhaust emission purifying apparatus for an engine which adopts a temperature sensitive urea sensor in the past filed Japanese Patent Application No. 2003-366737. In this apparatus, the urea concentration is detected by the urea sensor, and also, when a high concentration above a normal region is detected, a judgment is made that a residual amount of the urea water is deficient, whereas when a low concentration below the normal region is detected, a judgment is made that the urea water is in a dilute state equal to or near the water, and accordingly, the urea concentration is abnormal. When either one of the judgments is made, measures are taken to stop the urea water injection and the like. Further, in this apparatus, particularly in the latter case where the low concentration is detected, when this low concentration is repetitively detected for the predetermined number of frequencies, this low concentration is adopted as an established value so that the reliability of the detected concentration is ensured (refer to FIGS. 7 and 9 of the above prior application).
However, there is the following problem in such an exhaust emission purifying apparatus in which the detection of the concentration higher or lower than the normal region and the abnormality judgment corresponding to this detection are not performed simultaneously, and consequently, there appears a time lag between the former detection and the latter judgment. Namely, in the case where a driver inadvertently or intentionally replenishes the water or the diluted urea water after it is judged that the residual amount of the urea water is deficient, the judgment of the residual amount deficiency is cancelled, but the judgment of the abnormal concentration is made belatedly by a period of time corresponding to the above time lag, after the replenishment. Therefore, it is impossible to take the measures to the abnormal concentration during the time lag, and accordingly, there is a possibility that unpurified NO is discharged into the atmosphere.